Silica glass has conventionally been used in furnaces and heat treating jigs for the semiconductor industry because it has a high purity and an excellent heat resistance. It is important that the temperature distribution in the above heat treatment furnace is uniform. To achieve this, a reactor tube is produced from opaque silica glass containing 100,000 bubbles/cm.sup.3 or less, as found in Japanese Laid-Open Patent Publication No. H5 -900, or a heat shielding plate made of opaque silica glass containing less than 6,000 bubbles/cm.sup.3 is provided at both ends of a boat on which a semiconductor wafer has been placed, as described in Japanese Laid-Open Utility Model Publication No. H1-162234.
In recent years, vertical furnaces have become widespread in the semiconductor industry. However, with this vertical furnace, the lower end of the furnace is put on a metal frame and heat rays are irregularly diffused at the joint of the furnace and the metal frame. Alternatively, a cooling unit provided for protecting a sealing member which seals the joining part of the flange of the furnace to the metal frame disturbs the furnace temperature so that the temperature distribution in the furnace is not uniform. It has therefore been usual to provide light diffusing and thermal shielding plates.
Opaque silica glass has been preferred as the light diffusing and thermal shielding material described above because of its good heat resistance and thermal shielding. The light diffusing and thermal shielding members made of opaque silica glass described above are generally cut out from a solid opaque silica glass block for ease of production. Further, a method where a mold is filled with quartz raw material granules, and in particular, rock crystal granules, which is then sintered in an electric furnace has been used for producing opaque silica glass block. However, since large cavities are formed at the center of the block in this conventional method, a high purity opaque silica glass block containing uniform bubbles and having excellent light diffusing and thermal shielding could not be produced. In addition, the bubble density of the conventional opaque silica glass was less than 100,000 bubbles/cm.sup.3, as described in the above-mentioned Japanese Laid- Open Utility Model Publication No. H1-162234. Use of light diffusing members made of such opaque silica glass as light diffusing and thermal shielding members for vertical furnaces, which has recently become widespread in furnaces which are used for manufacturing semiconductors could not sufficiently prevent either irregular diffusion of heat rays at the joint of the flange of the vertical furnace and a metal frame, or deterioration of the furnace temperature due to a cooling unit provided for protecting the sealing member. Further, a conventional opaque silica glass member deforms significantly at high temperatures. Particularly, when heat-treating a silicon wafer up to 1000.degree. C. or higher, an extended heating time causes large thermal deformation and prevents the opaque silica glass from functioning as a thermal shielding and light diffusing member, thereby shortening the life time of the furnace.
Investigation by the present inventors in order to solve the problems described above has resulted in the findings that thermal deformation of opaque silica glass at high temperatures is related to the density of bubbles and heat resistance of the silica glass, that thermal deformation is decreased by decreasing the whole volume of the bubbles in the opaque silica glass while decreasing the diameter of the bubbles and increasing the bubble density to increase the whole bubble cross section per unit volume of the opaque silica glass, and further that the heat resistance is improved by reducing the concentrations of sodium, potassium and OH groups each contained in the opaque silica glass in specific ranges or lower. It has been found that nitrogen element doping within a specific range into the silica glass is effective particularly for improving heat resistance and that this is particularly notable when synthetic quartz grain is used for raw material grain.
It has also been found that the opaque silica glass described above can be produced by using quartz raw material grain having a specific particle size and sintering it in a mold at a specific temperature-increase speed. In such a method for producing the opaque silica glass, however, problems still remain in the case of a large scale opaque silica glass block that a large scale heating source is required and a loss of product in production becomes large as compared with an medium scale opaque silica glass block containing uniform fine independent bubbles in the bulk which can be efficiently produced in a conventional method, and the product loss increase, because a large difference in temperature occurs between the upper and lower end portions of a filling layer of quartz grain filled in a mold and there may be, therefore, a chance where the production process is over while the upper end portion is still left unmelted. In the case of producing such a large scale opaque silica glass block, a special technique of heating is applied to producing of the large scale opaque silica glass block with none of the faults mentioned above, said special technique being that a belt-like heating source is located perpendicular to the trunk of a filling layer of quartz grain in a heat-resistant mold in order to form a heating zone in the quartz grain-filling layer, the heating zone is upwardly moved from the lower end portion of the quartz grain or to the contrary, downwardly moved from the upper end porton in a successive manner. The present invention has been completed based on such findings.
According to the method of the present invention, an opaque silica glass having a large bubble density, and allowing independent bubbles having a small size uniformly dispersed is able to be produced and besides, a large scale opaque silica glass block is easy to be obtained. The best kinds of thermal shield and light diffusing members can be manufactured at a low cost by slicing the silica glass block.